This invention relates to an apparatus for positioning an object precisely in three linear and three rotary dimensions, with long travel being provided in two of the linear dimensions. The apparatus is particularly suited for use in extreme-UV projection lithography.
In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Typically, a xe2x80x9ctransparencyxe2x80x9d of the subject pattern is made having areas which are selectively transparent, opaque, reflective, or non-reflective to the xe2x80x9cprojectingxe2x80x9d radiation. Exposure of the coating through the transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing and has supplanted proximity printing. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a. Extreme UV) (wavelength range of xcex=100 to 200 xc3x85) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. With projection photolithography, a reticle (or mask) is imaged through a reduction-projection lens onto a wafer. Reticles for EUV projection lithography typically comprise a silicon substrate coated with an x-ray reflective material and an optical pattern fabricated from an x-ray absorbing material that is formed on the reflective material. In operation, EUV radiation from the condenser is projected toward the surface of the reticle and radiation is reflected from those areas of the reticle reflective surface which are exposed, i.e., not covered by the x-ray absorbing material. The reflected radiation effectively transcribes the pattern from the reticle to the wafer positioned downstream from the reticle. A scanning exposure device uses simultaneous motion of the reticle and wafer, with each substrate being mounted on a chuck that is attached to an X-Y stage platen, to continuously project a portion of the reticle onto the wafer through a projection optics. Scanning, as opposed to exposure of the entire reticle at once, allows for the projection of reticle patterns that exceed in size that of the image field of the projection lens. Mirrors are mounted along the sides of a stage platen; and interferometer heads that direct laser beams onto the associated mirrors and detect the beam reflection therefrom are employed for position measuring purposes. Movement of the stage platen is accomplished with motorized positioning devices. A stage platen similarly supports the wafer substrate.
Current positioning systems for wafer and reticle stages are composed of bearings and actuators. The types of bearings employed include, for example, sliding contact, rolling elements, air, hydrostatic, flexural, and magnetic. An actuator is a device that provides the accelerating force to move a body relative to another. The types of actuators employed include, for example, motors (linear and rotating), pneumatic, hydraulic, piezoelectric, and electromagnetic (Lorentz and variable reluctance). There are seemingly innumerable bearing and actuator design considerations that affect the performance of a precision machine. Some of the more crucial considerations for a lithography stage are the requirements of acceleration, stiffness, resolution, range of motion, accuracy, repeatability, and thermal performance. Lithography steppers currently produced use a combination of mechanical or air-bearing stages to achieve control of the wafer location in six degrees of freedom. There are various problems associated with these present-day positioning devices.
For mechanical stages with their sliding or rolling bearing elements, positioning errors are caused by friction and the ensuing particulate generation. Moreover, it is difficult to achieve rectilinear motion with flexural stages, this results in coupled degrees of freedom. These systems are also expensive and difficult to manufacture and they usually require electrodischarge machining (EDM), surface grinding, and hand-lapping processes. The resolution is limited by the bearing surface finish. Furthermore, the actuators used for the fine motions (e.g., piezo, voice coil and fluid filled bellows) can be unreliable. Each degree of freedom generally requires an additional layer of mechanical complexity and in some cases must be further subdivided into coarse motions and fine motions. These successive layers result in numerous uncontrolled, underdamped, rigid and elastic body resonant modes which limit allowable stage bandwidth.
With air bearing stages, air bearings are constrained to move in the near vicinity of a plane and thus an additional fine motion stage must be stacked on an air bearing to achieve six degrees of freedom. The fine motion stage must have sufficient travel to overcome any straightness errors present in the air bearing axis. It is difficult to use an air bearing in a vacuum environment; such environments are required in state of the art lithographic systems. In an air bearing, there are up to five uncontrolled degrees of freedom at the wafer. These rigid body modes can limit stage control bandwidth and stage settling times.
The aforementioned problems associated with these conventional stages have limited the ultimate speed and resolution that is attainable. One possible solution to achieve an increase in wafer throughput with a simultaneous improvement in resolution is through the use of a magnetic levitation stage, which is described, for instance, in U.S. Pat. Nos. 5,157,296, 5,196,745, 5,294,854 and 5,699,621.
With magnetic levitation a body is suspended through the interaction of magnetic fields. As it applies to precision motion control, magnetic levitation is used to suspend a rigid body and control its position and attitude in multiple degrees of freedom. Magnetic levitation stages have the unique property that the bearing and actuator functions are combined into a single element. This allows a single rigid body to be positioned and controlled without any mechanical contact. The forces used to perform these functions can be provided by either variable-reluctance or Lorentz force electromagnetic actuators.
A variable-reluctance actuator is a uni-polar forcing device. This means that the reluctance of the magnetic circuit consisting of the electromagnet, the gap, and the ferromagnetic target of the actuator, is minimized by reducing the gap spacing, consistent with external constraints. In other words, the highly permeable ferromagnetic actuator core, driven by a current conducting winding is attracted to a corresponding highly permeable ferromagnetic target. The advantage of this type of actuator is its extremely high force density (N/w). This translates into high servo bandwidths with excellent power efficiency. The disadvantages are that it requires two actuators to provide a bipolar force and the force is a non-linear function of the gap between the core and the target.
A Lorentz force actuator produces bi-polar force through the interaction of a magnetic field produced by current in a coil of wire and the field from a permanent magnet. Reversing the current polarity reverses the direction of the force. A Lorentz force actuator in its simplest form is a voice coil, and in a more complex arrangement is a linear motor. The advantages of linear motors are the extended range of travel over the motor surface, the high acceleration capability, and they are non-contact. The disadvantage is low force density. It requires a very large surface area to provide sufficient control effort with reasonable power dissipation.
The art is in search of improved magnetic levitation precision positioning systems for projection lithography.
The present invention is directed to a magnetic levitation stage that uses a combination of variable-reluctance and Lorentz force actuators in a power optimal configuration. The configuration presented is designed to provide a small footprint, be power optimal, and provide excellent scanning performance.
In one embodiment, the invention is directed to a stage for precise positioning of a chuck in three orthogonal linear axes and in three orthogonal rotation axes that includes:
(a) a first subassembly that includes:
(i) a monolithic mirror that supports the chuck wherein the monolithic mirror has at least two polished orthogonal faces for interferometric determination of the X, Y, and "THgr"z positions;
(ii) a plurality of electromagnetic actuators that control fine positioning in all six axes and coarse positioning in one axis;
(iii) a position sensor for measuring the vertical Z position of the monolithic mirror; and
(iv) a Lorentz actuator, that includes magnet array, for effecting motion in the Y axis; and
(b) a second subassembly that includes a stepping axis beam over which the first subassembly is suspended, wherein the stepping axis beam includes a drive coil array for the Lorentz actuator.
In the preferred embodiment, the second subassembly further includes a cable stage subassembly that is positioned a fixed distance away from the monolithic mirror.
In another preferred embodiment, the stage further includes a mechanical zero reference for the first subassembly.
In yet another preferred embodiment, the stage also includes a linear motor with independently controlled coils to enable independent control of the motion of the first and second subassemblies wherein each of the first and second subassemblies includes respective first and second magnetic arrays.
In another embodiment, the invention is directed to a stage comprising an actuator that includes a magnetic array that is mounted on a strain reducing flexure mount.